Stephen Jay Gould and Richard Lewontin used the architectural term spandrel (the triangular gap at the corner of an arch) to describe a byproduct of evolution. Basilica di San Marco, Venice
In evolutionary biology, a spandrel is a phenotypic characteristic that is a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive selection.
That is, it is a trait that is not particularly advantageous to have,
though it is retained because it is not particularly harmful to have.
The term "spandrel"
originated as an architectural word for the roughly triangular space
between the tops of two adjacent arches and the ceiling. These spaces
were not actually utilized until later on, when artists realized they
could make designs and paint in these small areas, enhancing the overall
design of the building.
Stephen Jay Gould and Richard Lewontin brought the term into biology in their 1979 paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme". This defined the biological concept and argued the case for a structuralist view of evolution.
Origin of the term
The term was coined by Harvard paleontologist Stephen Jay Gould and population geneticist Richard Lewontin in their paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" (1979). Evolutionary biologist Günter P. Wagner called the paper "the most influential structuralist manifesto".
In their paper, Gould and Lewontin employed the analogy of spandrels in Renaissance architecture: curved areas of masonry between arches supporting a dome
that arise as a consequence of decisions about the shape of the arches
and the base of the dome, rather than being designed for the artistic
purposes for which they were often employed. The authors singled out
properties like the necessary number of four spandrels and their
specific three-dimensional shape. At the time, it was thought in the
scientific community that everything an animal has developed that has a
positive effect on that animal's fitness was due to natural selection or some adaptation.
Gould and Lewontin proposed an alternative hypothesis: that due to
adaptation and natural selection, byproducts are also formed. These
byproducts of adaptations that had no real relative advantage to
survival, they termed spandrels. In the biological sense, a "spandrel"
might result from an architectural requirement inherent in the Bauplan of an organism, or from some other constraint on adaptive evolution.
Evolutionary biology uses the term spandrel for features of an
organism arising as byproducts, rather than adaptations, that have no
clear benefit for the organism's fitness and survival. In response to
the position that spandrels are just small, unimportant byproducts,
Gould and Lewontin argue that "we must not recognize that small means
unimportant. Spandrels can be as prominent as primary adaptations". A
main example used by Gould and Lewontin is the human brain. Many
secondary processes and actions come in addition to the main functions
of the human brain. These secondary processes and thoughts can
eventually turn into an adaptation or provide a fitness advantage to
humans. Just because something is a secondary trait or byproduct of an
adaptation does not mean it has no use.
In 1982, Gould and Vrba introduced the term "exaptation" for characteristics that enhance fitness in their present role but were not built for that role by natural selection.
Exaptations may be divided into two subcategories: pre-adaptations and
spandrels. Spandrels are characteristics that did not originate by the
direct action of natural selection and that were later co-opted for a
current use. Gould saw the term to be optimally suited for evolutionary
biology for "the concept of a nonadaptive architectural by-product of
definite and necessary form – a structure of particular size and shape
that then becomes available for later and secondary utility".
Criticism of the term
Gould and Lewontin's proposal generated a large literature of critique, which Gould characterised as being grounded in two ways. First, a terminological claim was offered that the "spandrels" of Basilica di San Marco were not spandrels at all, but rather were pendentives.
Gould responded, "The term spandrel may be extended from its particular
architectural use for two-dimensional byproducts to the generality of
'spaces left over', a definition that properly includes the San Marco
pendentives."
Other critics, such as Daniel Dennett, further claimed (in Darwin's Dangerous Idea
and elsewhere) that these pendentives are not merely architectural
by-products as Gould and Lewontin supposed. Dennett argues that
alternatives to pendentives, such as corbels or squinches, would have served equally well from an architectural standpoint, but pendentives were deliberately selected due to their aesthetic value. Critics such as H. Allen Orr
argued that Lewontin and Gould's oversight in this regard illustrates
their underestimation of the pervasiveness of adaptations found in
nature.
Ian Kluge
criticizes the whole subject of spandrels to be bogged down in a
definitional debate. He argues it is not entirely clear what is and is
not a spandrel. He also argues all examples of spandrels, pendentives,
corbels and squinches do actually serve a function; they are necessary
to achieve something, but that necessity is exactly what
epiphenomenalism denies.
Response to criticism
Gould responded that critics ignore that later selective value is a separate issue from origination
as necessary consequences of structure; he summarised his use of the
term 'spandrel' in 1997: "Evolutionary biology needs such an explicit
term for features arising as byproducts, rather than adaptations,
whatever their subsequent exaptive utility... Causes of historical
origin must always be separated from current utilities; their conflation
has seriously hampered the evolutionary analysis of form in the history
of life." Gould cites the masculinized genitalia of female hyenas and the brooding chamber of some snails as examples of evolutionary spandrels.
Gould (1991) outlines some considerations for grounds for
assigning or denying a structure the status of spandrel, pointing first
to the fact that a structure originating as a spandrel through primary
exaptation may have been further crafted for its current utility by a
suite of secondary adaptations, thus the grounds of how well crafted a
structure is for a function cannot be used as grounds for assigning or
denying spandrel status. The nature of the current utility of a
structure also does not provide a basis for assigning or denying
spandrel status, nor does he see the origin of a structure as having any
relationship to the extent or vitality of a later co-opted role, but
places importance on the later evolutionary meaning of a structure. This
seems to imply that the design and secondary utilization of spandrels
may feed back into the evolutionary process and thus determine major
features of the entire structure. The grounds Gould does accept to have
validity in assigning or denying a structure the status of spandrel are
historical order and comparative anatomy.
Historical order involves the use of historical evidence to determine
which feature arose as a primary adaptation and which one appeared
subsequently as a co-opted by-product. In the absence of historical
evidence, inferences are drawn about the evolution of a structure
through comparative anatomy. Evidence is obtained by comparing current
examples of the structure in a cladistic context and by subsequently trying to determine a historical order from the distribution yielded by tabulation.
Language and music as spandrels
Linguist Noam Chomsky has argued that the "language faculty", and the property of discrete infinity or recursion that plays a central role in his theory of universal grammar
(UG), may have evolved as a spandrel. In this view, Chomsky initially
pointed to language being a result of increased brain size and
increasing complexity, though he provides no definitive answers as to
what factors led to the brain attaining the size and complexity of which
discrete infinity is a consequence. Steven Pinker and Ray Jackendoff say Chomsky's case is "unconvincing" and that "language maps among
recursive systems rather than being a straightforward externalization
of a single recursive system", and as an example, numerical recursion
"is parasitic on language (rather than vice versa)" among other
arguments. Pinker contends that the language faculty is not a spandrel, but rather a result of natural selection.
Newmeyer (1998) instead views the lack of symmetry, irregularity and
idiosyncrasy that universal grammar tolerates and the widely different
principles of organization of its various sub-components and consequent
wide variety of linking rules relating them as evidence that such design
features do not qualify as an exaptation. He suggests that universal
grammar cannot be derivative and autonomous at the same time, and that
Chomsky wants language to be an epiphenomenon and an "organ"
simultaneously, where an organ is defined as a product of a dedicated
genetic blueprint.
Rudolph Botha counters that Chomsky has offered his conception of the
feature of recursion but not a theory of the evolution of the language
faculty as a whole.
Pinker has written that "As far as biological cause and effect
are concerned, music is useless. It shows no signs of design for
attaining a goal such as long life, grandchildren, or accurate
perception and prediction of the world", and "I suspect that music is
auditory cheesecake, an exquisite confection crafted to tickle the
sensitive spots of at least six of our mental faculties."
Dunbar found this conclusion odd, and stated that "it falls foul of
what we might refer to as the Spandrel Fallacy: 'I haven't really had
time to determine empirically whether or not something has a function,
so I'll conclude that it can't possibly have one.'"
Dunbar states that there are at least two potential roles of music in
evolution: "One is its role in mating and mate choice, the other is its
role in social bonding."
